Many potential applications have emerged for metallic oxide superconductors that exhibit superconductivity at relatively high temperatures, e.g., temperatures that can be maintained via liquid nitrogen cooling. For many such applications, e.g., applications involving the production, distribution, and utilization of electric power, it is desirable to provide superconductive bodies having elongated shapes, for example wires or rods comprising metallic oxide superconductor material.
A number of investigators have attempted to use wire-drawing or extrusion techniques to produce rods or wires comprising metallic oxide superconductor material. For example, M. R. Notis, et al., "Fabrication and Characterization of Ceramic Superconducting Composite Wire," in Advances in Superconductivity, Proc. 1st Int. Symp. on Superconductivity (1989) pp. 371-375, and M-S Oh, et al., "Fabrication and Microstructure of Composite Metal-Clad Ceramic Superconducting Wire," J. Am. Ceram. Soc. 72 (1989) pp. 2142-2147, have reported the use of wire drawing to achieve areal reduction ratios (also referred to as "extrusion ratios") up to about 1.4 for Ba.sub.2 YCu.sub.3 O.sub.7 cores surrounded by silver claddings or composite claddings of silver and stainless steel. These authors also suggested, without providing any guidance, that hydrostatic extrusion may be used as an alternative method to wire drawing. For many applications, however, it is desirable to achieve extrusion ratios greater than 1.4. Moreover, these authors observed displacement reactions occurring between the core material and the cladding material. Such reactions are undesirable because they may degrade the performance of the superconductive core.
Other investigators have employed extrusion techniques. For example, S. K. Samanta, et al., "Manufacturing of High T.sub.c Superconducting Ceramic Wires by Hot Extrusion," Annals of the CIRP 37 No. 1 (1988) pp. 259-261, reported an extrusion ratio of 9 achieved by means of conventional (i.e., non-hydrostatic) extrusion of metal-clad powder at temperatures of 825.degree. C. and 895.degree. C. R. N. Wright, et al., "Deformation Processing of High T.sub.c Superconducting Wire," in Processing and Applications of High T.sub.c Superconductors, W. E. Mayo, ed., The Metallurgical Society (1988) pp. 139-150, reported extrusion of metal-clad powder at even higher extrusion ratios of 15 at an extrusion temperature of 850.degree. C. However, extrusion at temperatures substantially greater than about 800.degree. C., and even at temperatures substantially greater than about 450.degree. C., is not generally desirable because, inter alia, at least some commonly used hydrostatic fluids for pressure distribution during hydrostatic extrusion are difficult or impossible to use at such temperatures.
Investigators have also sought to achieve relatively high extrusion ratios at relatively low extrusion temperatures. P. J. McGinn, et al., "Texture Processing of Extruded YBa.sub.2 Cu.sub.3 O.sub.6+x Wires by Zone Melting," Physica C 165 (1990) pp. 480-484, and P. J. McGinn, et al., "Zone Melt Texturing of YBa.sub.2 Cu.sub.3 O.sub.6+x with Silver Additions," Physica C 167 (1990) pp. 343-347, reported cold extrusion of powdered Ba.sub.2 YCu.sub.3 O.sub.7 material mixed with organic solvent, binder, dispersant, and plasticizer. However, it is generally believed that the organic materials must be removed from the core in order to achieve a useful superconductive article. Thus, for example, R. N. Wright, et al., cited above, reported achieving extrusion ratios of 10, and even of 70, by cold extrusion of core material comprising 55 vol. % superconductor powder and 45 vol. % polyethylene spheres. It was observed that the extrudate was not electrically continuous, but could be made superconducting by extracting the polyethylene. However, carbon-containing residues are believed capable of degrading the superconductive properties of the core material. Moreover, removal of the polyethylene involves heating an unencapsulated core. As an unintended side effect, it is possible for the stoichiometry of the core material to be changed, for example by oxygen evolution. As a consequence, extrusion techniques that involve organic additives may be unacceptable for at least some applications.
Still other investigators have attempted warm extrusion without organic additives. S. Samajdar, et al., "A Phenomenological Model On The Deformation Mechanism Of YBa.sub.2 Cu.sub.3 O.sub.7-x +Ag.sub.2 O Composite," J. Mat. Sci. Lett. 9 (1990) pp. 137-140, and S. K. Samanta, et al., "A Novel Processing Technique For Fabrication of Flexible YBa.sub.2 Cu.sub.3 O.sub.7-x Wire," J. Appl. Phys. 66 (1989) pp. 4532-4534, have reported extrusion, at 450.degree. C., of Ba.sub.2 YCu.sub.3 O.sub.7 powder containing 50-70 vol. % silver oxide (Ag.sub.2 O). An extrusion ratio of 9 was reportedly achieved. However, the presence of substantial quantities of non-superconductive material in the core may limit the critical current density of the (superconducting) core, and may even threaten the electrical continuity of the core. Thus it is desirable to have a core containing less than about 30 vol. % non-superconductive material.
Thus, investigators have sought, hitherto without success, a method for extrusively forming, at temperatures below about 800.degree. C., and especially at temperatures below about 450.degree. C. (exclusive of adiabatic heating during extrusion), elongated bodies that comprise metallic oxide superconductor material that is substantially free of organic additives, and is diluted by less than about 30 vol. % non-superconductor material. Investigators have also sought, hitherto without success, an extrusive method for forming, at such relatively low temperatures, bodies that are substantially free of elemental carbon or organic additives, and that have experienced an extrusion ratio greater than about nine.